Impacting of metal which has been cooled below its ductile-brittle transition temperature has been visualized by the prior art as a means to produce metal powder. One of the first methods of the prior art was to precondition the metal by collecting the scrap in bales and immersing the bales in liquid nitrogen. This first method and others to date implementing this concept have not been completely successful. It has been found particularly difficult to comminute metal to a uniform powder product having a particle size of less than -30 mesh. It is difficult to achieve this type of product, starting with metal scrap of a size greater than 1/2 inch as its smallest transverse dimension, because repeated impacting, necessary to properly fragment the starting material contributes heat energy causing at least local portions of the pieces to become ductile. It has been found most difficult to maintain a temperature in the range of -95.degree. to -110.degree. F for mild carbon steel since the metal elements (milling elements) producing fragmentation impart considerable chloric heat content to the metal pieces being fragmented; the pieces also are exposed to non-supercooled portions of the milling chamber for momentary periods.
It has also been difficult to cryogenically process the metal scrap so that it would have satisfactory compressibility in the powder form when employing the powder in conventional metallurgy techniques.